1. Field of the Invention
This invention concerns an improvement in a telescopic arm comprising a slide arm engaged slidably with a base arm which is pivoted to a boom or the like on the main body of a civil engineering machine and, more particularly, it relates to a telescopic arm in which the slide arm and the base arm are caused to slide with each other in a plane-to-plane engagement and the portion for the sliding engagement situated near the forward end that is put under the effect of earth and sand is formed at an inner position from the surface on the side of the slide arm.
2. Description of the Prior Art
As a device for varying and widening the working range of an operation arm attached to a civil engineering machine such as an excavator, there has been known an assembly referred to as a telescopic arm. In the telescopic arm, a base arm connected to the top end of the boom of the machine and a slide arm are slidably engaged with each other by means of guide plates disposed at the lower surface of the base arm and at the upper surface of a slide arm on both ends respectively such that the guide plates of the slide arm are held between upper and lower rollers attached at the forward end of the base arm, while the guide plates of the base arm are held between upper and lower rollers attached at the rearward end of the slide arm.
However, since the working loads upon excavation are exerted on the pins for the rollers, the loads are concentrated to the pins thereby resulting in problems in view of the strength that they are readily deformed or flexed.
Further, since the telescopic arm of this type is extended or contracted by the rolling of the rollers along the guide rails exposed to the outside, earth and sand or concrete lump deposited on the rail surface may possibly get into the rollers thereby also causing deformation and flexure in the roller shafts. Further, the axes for the entire arm tends to be distorted to hinder the smooth telescopic operation.
Particularly, since the rails of the slide arm at the forward portion are protruded outwardly, they are readily deformed by external objects. In addition, even if the deformation is resulted only locally, since the rollers rotate in point-to-point contact with the deformed surface, undesired effects are also caused to other members such as wearing or localized abrasion of the rollers, thereby making the maintenance work extremely difficult.